The present invention relates to a laminated filter and a duplexer used mainly for a radio frequency device such as a portable telephone and the like, and a mobile communication apparatus using the same.
A laminated filter of the prior art generally comprises dielectric layers 1401a, 1401b, 1401c, 1401d and 1401e, resonator electrodes 1402a and 1402b, load capacitor electrodes 1403a and 1403b, an inter-resonator coupling capacitor electrode 1404, input/output coupling capacitor electrodes 1405a and 1405b, and shielding electrodes 1406a and 1406b, as shown in FIG. 14A.
Ends of the electrodes 1402a and 1402b, and the electrodes 1406a and 1406b are connected to a grounding terminal electrode 1408a provided on a side surface of a dielectric, and, ends of the electrodes 1403a and 1403b, and the electrodes 1406a and 1406b are connected to a grounding terminal electrode 1408b on another side surface of the dielectric. The electrode 1405a is connected to an input/output terminal electrode 1407a provided on a side surface of the dielectric, and the electrode 1405b is connected to another input/output terminal electrode 1407b provided on another side surface of the dielectric. The electrodes 1408a and 1408b are grounded to constitute a structure.
Each of the electrodes in the above-described laminated filter functions as a stripline in a microwave band for which this laminated filter is used, since the electrodes are formed in the dielectric. Therefore, an equivalent circuit of this laminated filter is represented by FIG. 14B in the microwave band. In FIG. 14B, inductors 1613 and 1615, respectively, represent inductance components of the electrodes 1403a and 1403b. An inductor 1606 represents an inductance component of the electrode 1404. Furthermore, inductors 1603 and 1609 represent inductance components of the electrodes 1405a and 1405b, respectively.
In the above structure, the electrodes 1402a and 1402b act as quarter-wave resonators, since they are grounded at one end. Moreover, because the electrode 1404 and the electrodes 1402a and 1402b, as well as the electrodes 1405a and 1405b and the electrodes 1402a and 1402b compose parallel plate capacitors between them, they provide capacitive couplings between input/output terminals and the resonators, and also between the resonators. Furthermore, an attenuation pole (a frequency at which an impedance between the input/output terminals increases) can be formed in a transmission characteristic with an electromagnetic coupling obtained by adjusting widths of and a space between the electrodes 1402a and 1402b, and a capacitance obtained by adjusting the parallel plate capacitors formed between the electrodes 1404, and 1402a and 1402b. 
As a result, the attenuation pole is formed at one side of a pass band 1701 in the transmission characteristic between the input/output terminals, as shown in FIG. 14C, thereby serving as a band-pass filter having an attenuation band 1702 in vicinity of the pass band 1701.
In addition, a duplexer of the prior art comprises a receiving filter 1501, a transmission filter 1502, and a phase-shifting circuit 1503, as shown in FIG. 15, and one end of the receiving filter 1501 serves as a receiving terminal 1510, and one end of the transmission filter 1502 as a transmission terminal 1511.
The phase-shifting circuit 1503 comprises an inductor 1504, another inductor 1505, a capacitor 1506, a capacitor 1507, and another capacitor 1508. In the duplexer, the capacitor 1506, the inductor 1504, and the capacitor 1507 are designed to become equivalent to a transmission line, which is approximately one quarter of a wavelength at a pass band frequency of the transmission filter 1502. The capacitor 1507, the inductor 1505, and the capacitor 1508 are also designed to become equivalent to a transmission line, which is approximately one quarter of a wavelength at a pass band frequency of the receiving filter 1501.
Of a transmission signal input from the transmission terminal 1511, only a signal component having the pass band frequency passes through the transmission filter 1502, and it is fed to the phase-shifting circuit 1503. The receiving filter 1501, as observed from a common terminal 1509, shows high impedance in this case, and thereby the transmission signal is output from the common terminal 1509 without flowing into a path toward the receiving filter 1501. On the other hand, a receiving signal input from the common terminal 1509 is fed to the phase-shifting circuit 1503. However, the signal is input only to the receiving filter 1501 without flowing into a path toward the transmission filter 1502, since an impedance as observed from the common terminal 1509 toward the transmission filter 1502 side is high in this case, and therefore the signal is output to the receiving terminal 1510 only after a signal component having the pass band frequency of the receiving filter 1501 passes through.
Consequently, the transmission signal input from the transmission terminal 1511 is output from the common terminal 1509 via the phase-shifting circuit 1503 without being influenced by the receiving filter 1501. The receiving signal input from the common terminal 1509 is also output to the receiving terminal 1510 via the phase-shifting circuit 1503 without being influenced by the transmission filter 1502. Hence, the device functions as a duplexer.
The laminated type filter of the prior art had a problem that it needs to increase a number of resonators in order to gain a magnitude of attenuation, thereby resulting in a large size and an increase of an insertion loss in the pass band.
Moreover, the duplexer of the prior art also had a problem in that it needs a phase-shifting circuit consisting of an inductor and a capacitor of chip components, thereby requiring a large area of mounting surface.
The present invention is intended to address the above problems, and it aims at realizing a laminated filter having a low insertion loss and a high attenuation with a simple structure, and a duplexer of a small size with a small number of components.
In a laminated filter having a plurality of resonator electrodes, an inter-resonator coupling capacitor electrode for coupling between adjacent resonators, and two input/output coupling capacitor electrodes for coupling between input/output terminals and resonator electrodes, the present invention is to provide a capacitor electrode for electrically connecting one side of the input/output terminals with a portion of the input/output coupling capacitor electrode, wherein the input/output coupling capacitor electrode and the capacitor electrode comprise a parallel circuit.
This composition forms a parallel resonance circuit in one of the input/output terminals, and provides an additional attenuation pole besides another attenuation pole formed with an electromagnetic coupling between the resonators and an inter-resonator capacitance, thereby realizing the laminated filter of a high magnitude of attenuation with the same shape as that of the prior art.
Moreover, in a laminated filter having a pass band in a first band, and an attenuation band in a second band, there is provided a parallel circuit as described above at one side of the input/output terminals, whereby an attenuation pole formed by the parallel circuit is set in the vicinity of the second band. Furthermore, in a laminated filter having an attenuation band in the first band and a pass band in a second band, there is provided a parallel circuit as described above at one side of the input/output terminals, whereby an attenuation pole formed by the parallel circuit is set in the vicinity of the first band. A duplexer of the present invention is composed by connecting these two laminated filters at the input/output terminals where the parallel circuits are provided, and using the connected point as a common terminal.
With the described structure, the duplexer can be realized without using a phase-shifting circuit, since majority of a signal component passing through either one of the laminated filters is input to the common terminal because the parallel circuit of the other laminated filter provides a high impedance.